Soft devices are machines built from soft materials (e.g., elastomers, gels, liquids). These soft devices are useful for their ability to change their size and shape readily upon electrical, chemical, pneumatic, ferrofluidic, or hydraulic actuation. In addition, the low stiffness of the elastomeric materials used to construct these devices (Young's modulus <10 MPa) enables them to deform readily in response to external forces. These attributes allow soft devices to perform functions that are challenging for hard machines. Examples include interacting with delicate, soft materials (e.g., biological tissues), and performing unstructured tasks (e.g., gripping objects of undefined shape). Machines, whether they are hard or soft, typically require the integration of electrical components (e.g. motors, sensors, microcontrollers, displays, pumps, batteries, etc.) in order to perform sophisticated tasks. These devices must be controlled in order to create an autonomous or semi-autonomous soft robotic system.
Knowing the morphology of a soft actuator is important for making a control system for a soft robot. This is because, unlike a hard robot, a soft robot can change volume and shape based on pneumatic or hydraulic inflation pressure or by forces in the external environment. In addition, unlike a hard robot, the response of the soft material of the actuator to force, whether external or internal, is highly non-linear making calculations that predict the behavior of the actuator in response to force very complex and difficult.
Having to know the morphology of the robot is an emergent problem that was not as prominent in the world of conventional hard robots. In a hard robot, force from the external environment generates a simpler outcome. For example, force applied to a hard robotic arm will move the arm a fixed distance that is easy to calculate since the robot is made from a series of hard components and linkages that do not deform during standard operation. In contrast, when force from the external environment is applied to a soft robotic arm, one gets a very complex outcome since the soft arm will both move and deform.
Additionally, the stiffness of the elastomer that makes up the actuator may change during actuation. For example, if the inflation pressure is at 30% of the max inflation pressure of an actuator, the elastomer is in a low strain state where the elastomer has stiffness “A”; and when the inflation pressure is at 80% of the max inflation pressure, the elastomer is in a higher strain state with a different stiffness “B”. As a result, a different amount of force is required to achieve each increment of actuation.
Due to the intrinsic properties of elastomers, the stress vs. strain profile can be different for extension and relaxation. Elastomers show a high degree of hysteresis during cycles of loading and unloading. This discrepancy between the loading and unloading profile will change depending on how fast one cycles between the two. So as a result the system has memory. This aspect of elastomers will make soft actuators difficult to control using just the knowledge of the inflation pressure of the actuator. See, also, http://www.s-cool.co.uk/a-level/physics/stress-and-strain/revise-it/stress-strain-graphs.